For example, a value of a transmission ratio may be useful for controlling a vehicle, such as an automobile. Generally, the transmission ratio value may be obtained by the following methods. The first method is called a “Transmission Ratio Embedding Method”, and the second method is called a “Rotational-speed Ratio Calculating Method.”
The Transmission Ratio Embedding Method embeds values (in this case, gear reduction ratios or transmission ratios) as a control parameter into a control program, and selects one of the transmission ratio values based on a value detected by a gear position sensor. The Rotational-speed Ratio Calculating Method is disclosed, for example, in Japanese Unexamined Patent Publication No. 2002-201973, in which a value of the transmission ratio is calculated based on a rotational speed of a driving source (for example, a rotational speed of a crankshaft of an internal combustion engine), and a vehicle traveling speed (for example, a rotational speed of a speed sensor shaft). The entire disclosure of Japanese Unexamined Patent Publication No. 2002-201973 is incorporated herein by reference.
However, the Transmission Ratio Embedding Method may require a correction of the control parameter when an actual transmission ratio is changed by, for example, a change in design of the transmission. Further, the Rotational-speed Ratio Calculating Method tends to output a value that can be unstable because of, for example, a slight change in the transmission ratio by backlash of gears, a damping effect of a damper element (e.g., a coupling damper), etc.